Effect of Ni content and Hf addition on the unlubricated wear performance of Ni-rich NiTi alloys

Due to the combination of high hardness, high strength, excellent corrosion resistance, superelasticity and low elastic modulus, Ni-rich NiTi alloys have recently been considered as a promising alloy system for using in bearing and related applications. In this study, in order to examine the effects of the Ni content and Hf addition on the unlubricated wear performance of the Ni-rich NiTi alloys, both 52Ni, 55Ni, 52Ni-5Hf and 55Ni-5Hf (at%) alloys were fabricated. The microhardness test results indicate that 55Ni and 55Ni-5Hf samples exhibited higher hardness than the 52Ni and 52Ni-5Hf samples, respectively, this is attributed to the higher volume of the Ni₄Ti₃ phases in the higher Ni-rich alloys. In addition, the Hf addition further improves the hardness of the alloys. The wear test results also confirm the superior wear performance of the 55Ni-5Hf alloys than that of the 55Ni and 52Ni-5Hf alloys. The SEM observations from the worn surfaces demonstrated the same wear mechanisms of these tested alloys, which are mainly abrasive wear. Hence, it can be deduced that both the higher Ni content and Hf addition are beneficial to obtain better wear performance.